The aircraft propulsion system design process seeks to achieve the best overall system-level performance, weight, and cost for a given propulsion design. For example, current systems over the last 50-60 years have been finding ways to improve the efficiency of the aircraft propulsion system. Currently, these improvements result in a very small (e.g., less than 1%) increase in engine efficiency/performance. Further, the cost to reach such a small increase is substantial.
Still further, more and more electric aircraft technology that use electric actuation for flight control surfaces, and 270V dc for the power system have suffered problems due to regenerative energy that occurs when the electric actuator is back driven by the airstream. This causes regenerative energy to be produced by the electric actuator and increases the bus voltage unless absorbed or dissipated. In order to handle this problem clamping resistors are used to dissipate the energy, but must be cooled by emersion in the fuel tank. Emersion in the fuel tank then requires fuel reserves, limiting the mission capability. One alternative is to use batteries and supercapacitors that could recover the energy and reuse as peaking power. This, however, requires significant additional weight for the power electronics.